The advantages of dual chamber synchronous pacing are well recognized in the art. Pacemakers that can operate in one or more synchronous modes are able to take advantage of the heart's natural atrial, or sinus-activated, depolarizations. This allows the atrium to control the ventricular response rate where appropriate, and offers the improvement in ejection fraction that results from synchronizing the ventricular pacing stimulation with the atrial contraction. Prior studies have shown that the improvement in ventricular output due to synchronous pacing is most pronounced at relatively low heart rates. At higher heart rates the increase in cardiac output is largely due to the increased rate of contractions, but at lower rates there is a significant increase in efficiency that results from synchronous action. Accordingly, while the AV interval can be controlled to vary with rate, the most important value of the AV=f(r) curve is the lower rate limit (LRL). Available modes of pacing that provide synchronous pacing include VDI, VDD, DVI, DDD and DDD as well as rate-responsive variations thereof, among others.
The pacemaker art has also come to include multi-mode designs having the capability to which modes in response to changing patient conditions. Most dual and multi-chamber pacemakers are programmable to distinct modes, or are configured to switch automatically from one mode to another under certain prescribed conditions. See, for example, U.S. Pat. No. 4,527,568 and U.S. Pat. No. 4,920,965. But as a general rule it is advantageous to operate in a synchronized mode as much as possible wherein an atrial sense (AS) or delivered atrial pace pulse (AP) is followed (in the absence of a natural ventricular contraction) by a ventricular pace pulse (VP) that is timed to occur at a predetermined AV interval, or delay following the atrial event.
It is known that it is desirable to optimize the AV delay (AV), and also to set AV as a function of rate. See U.S. Pat. No. 5,330,511, Boute, assigned to the assignee of this invention and incorporated herein by reference in its entirety. The prior art shows a number of examples of pacemakers that attempt to adjust AV as a function of a sensed variable. See U.S. Pat. No. 4,303,075, wherein AV delay is modified in accordance with a sensed measure of stroke volume. U.S. Pat. No. 5,713,930 discloses setting AV delay by monitoring QT interval at different values of AV to determine the AV delay that corresponds to ventricular fusion, and then setting AV to a value just less than that corresponding to fusion. The pacemaker of the above-mentioned Boute patent optimizes AV by monitoring QT interval (QT) at different values of AV while pacing at LRL, and selecting the AV that corresponds to the longest QT. The following Table 1 lists patent references relating to the subject matter of this invention:
TABLE 1PATENT NO.INVENTOR(S)ISSUE DATEU.S. Pat. No. 4,303,075Heilman et al.Dec., 1981U.S. Pat. No. 5,330,511BouteJul. 19, 1994U.S. Pat. No. 5,534,016BouteJul. 9, 1996U.S. Pat. No. 5,560,368BergerOct. 1, 1996U.S. Pat. No. 5,713,930ven der Veen et alFeb. 3, 1998
All patent references listed in Table 1 above are hereby incorporated by reference in their respective entireties. As those of ordinary skill in the art will appreciate upon reading this Summary of Invention, Detailed Description and claims as set forth below, many of the devices and methods disclosed in the references of Table 1 may be modified advantageously by using the teachings of the present invention.
The above-listed U.S. Pat. No. 5,330,511 purports to base AV optimization on examining QT behavior at different AV intervals. This technique involves finding a value of QT at each AV test value, and selecting as optimal the AV that corresponds to the longest QT. See FIG. 6A for a representation of this aspect of the prior art. However, this test depends upon finding a stable QT at each AV test value, and this is difficult in practice. This reference notes that a three-minute waiting period is required after each change to a new AV level in order to let QT stabilize, and that fact alone makes for a lengthy test where up to 12 different AV values are tested. Further, it has been found that the QT interval remains sufficiently unstable even after three minutes to enable finding a value of QT precisely enough to be able to compare the QT values corresponding to different AV test values and determine which one is the maximum QT. Such instability may be the result of slow catecholamine feedback or other mechanisms. Thus, while the optimum value of AV is indeed that where QT is a maximum, in practice this is difficult to measure.
There thus remains a need in the art for a more reliable and precise method for measuring optimum AV interval for use in an implanted cardiac pacing device. There is a need for an improved system and method of periodically determining the optimum relationship between AV interval and pacing rate for a patient at one or more low pacing rates, and for making this determination reliably and in much less time than prior art methods. Also, it is desirable to utilize a parameter such as QT, or another measure of PQRST cardiac-cycle signals to obtain the needed information, so that no extra sensor has to be employed.